The present invention relates to an amorphous silicon film forming apparatus for forming an amorphous silicon film on a substrate, and more specifically to an improvement of an amorphous silicon film forming apparatus which forms an amorphous silicon film on a filming object (substrate) by introducing a gaseous silicon-containing compound into a reaction chamber and causing a glow discharge.
Presently, amorphous silicon (hereafter referred to as a-Si) is being used as an excellent photoconductive material to be applied to photoelectric conversion members, solar batteries, photosensitive materials for electrophotography, film transistors, etc. Many a-Si films have already been put to practical use.
The a-Si has advantages such that it can be formed into a structure of a wider area than single crystal, and its film can be formed on filming objects (substrates) of various shapes. Therefore, the a-Si is expected to be further developed and applied to various uses.
Existing methods for forming a-Si films include the high-frequency glow discharge method, reactive sputtering method, CVD method, etc. In a current method using high frequency glow discharge, a-Si films are formed by decomposing a gas containing silicon atoms as the raw gas, e.g., SiH4, Si2H6 or other silicon hydride gas.
FIG. 1 shows a prior art amorphous silicon film forming apparatus using the aforesaid method. In FIG. 1, numeral 1 designates a vacuum reaction vessel. A base 3 is disposed at the bottom portion of a reaction chamber 2 of the vacuum reaction vessel 1. A facing electrode 5 is attached to the ceiling of the vacuum reaction vessel 1 through the medium of an electrical insulator 4.
A conductive substrate 6 as a filming object set on the base 3 can be heated by a heater 7.
The facing electrode 5 is connected to a power source, such as a high-frequency power source or D.C. power source, for causing electrical discharge while the conductive substrate 6 is grounded.
Also, the vacuum reaction vessel 1 is connected individually with a high-vacuum exhaust system 10, a large-flow exhaust system 12, and a material gas supply system 14 through a valve 9, a gas pressure regulating valve 11, and a gas valve 13, respectively.
In forming a film, the gas valve 13 and the gas pressure regulating valve 11 are closed in advance. Then, the valve 9 is opened, and the vacuum reaction vessel 1 is evacuated to 10.sup.-6 torr or thereabouts by means of the high-vacuum exhaust system 10 using a diffusion pump, rotary pump (not shown), etc. At this time, the conductive substrate 6 is adjusted to a predetermined temperature between 150.degree. C. and 300.degree. C. by the heater 7.
Then, the gas valve 13 is opened, and a gaseous silicon-containing compound, such as SiH4 or Si2H6 gas (or a mixture of the gaseous silicon-containing compound and B2H6, PH3 gas, or other gas used as required, or a mixture of the gaseous silicon-containing compound and a gaseous carbon-containing compound, gaseous nitrogen-containing compound or gaseous oxygen-containing compound) is introduced as a material gas 15 into the vacuum reaction vessel 1. At the same time, the valve 9 is closed, and the gas pressure regulating valve 11 is opened, so that the exhaust system is switched from the high-vacuum exhaust system 10 including the diffusion pump, rotary pump, etc., to the large-flow exhaust system 12 including a mechanical booster pump, rotary pump (not shown), etc.
Subsequently, the gaseous silicon-containing compound and/or other doping gases are regulated by a flow controller (not shown) for a predetermined flow rate. Meanwhile, the pressure inside the vacuum reaction vessel 1 is adjusted to a predetermined value between 0.1 torr and 1.0 torr by operating the valve 11 connected to the mechanical booster pump.
Thereafter, a high-frequency electric power from the power source 8, with a frequency of 13.56 MHz and ranging from 10 W to 1 kW, is applied between the substrate 6 and the facing electrode 5 opposed thereto. As a result, glow discharge is produced between the base 3 and the facing electrode 5.
Thus, a plasma of gaseous silicon-containing compound or a mixture of the gaseous silicon-containing compound and the doping gas is produced, and an a-Si film starts to be formed on the substrate 6.
Radicals of the gaseous silicon-containing compound and/or other gases that are not conductive to the formation of the film are discharged through the large-flow exhaust system 12, passed through a combustion tower and a water scrubber, and then discharged into the open air.
In this film forming process using the glow discharge, the radicals of the gaseous silicon-containing compound could possibly undergo polymerization to produce by-products from silicon powder, depending on the film forming condition.
Hereupon, in forming an a-Si film for a photosensitive material for electrophotography, the film thickness need at least be 15 microns. For reasonable mass production, therefore, it is necessary to increase the film forming speed.
In order to speed up the film forming operation, it is usually necessary to raise the pressure inside the vacuum reaction vessel 1 and to apply greater high-frequency output to the facing electrode 5. However, by film forming under high-pressure, high-output conditions would produce plenty of by-products from the silicon powder. For example, the by-products from the silicon powder are intensively attached to portions A indicated by the dotted areas in FIG. 1. These by-products of the silicon powder if produced in bulk would clog the exhaust system and be trapped in the a-Si film being formed, greatly lowering the film quality. In mass-producing a-Si films, moreover, these by-products would involve many problems. For example, they would require the interior of the vacuum reaction vessel 1 to be cleaned for a long time with every cycle of the film forming operation.
Meanwhile, the film forming speed is limited within a range of about 6 to 8 microns/hour even though the pressure inside the vacuum reaction vessel 1 and the applied high-frequency output are both high. Thus, the mere formation of the a-Si film for the electrophotographic photosensitive material requires at least three hours. Considering the time required for evacuation, raising the substrate temperature, and cooling after film forming, a single cycle of the film forming operation would require about six hours in total. This should constitute a substantial hindrance to mass production.